In a transducer, energy of one form is converted to energy of a different form. Electroacoustic transducers particularly convert electrical signals to acoustic waves that may be perceived as audible sound to listeners. Some such electroacoustic transducers include horn drivers which produce sound pressure waves generated by a vibrating diaphragm—for example, a compression driver having an attached horn. Typically, the diaphragm of a compression driver is acoustically coupled to a horn via a phasing plug. The diaphragm and phasing plug are separated by a thin layer of air referred to as a compression chamber. The phasing plug performs several functions. The overall area of its acoustic entrance is significantly smaller than the area of a proximate diaphragm. This area gradually increases and matches the throat area of the waveguide or horn attached to the exit of the compression driver. The fact that the phasing plug entrance area is smaller than the area of the diaphragm increases loading impedance to provide matching of the output impedance of the vibrating diaphragm and the input impedance of the phasing plug followed by the horn or waveguide. Matched impedances provide maximum efficiency in the compression driver. Second, acoustic channels of the phasing plug provide equal path-lengths extending from different parts of the compression chamber to an exit of the phasing plug, the exit being coupled to an entrance (e.g., throat) of the horn. This prevents differences in phases of acoustic waves propagating through individual acoustic channels in the phasing plug and accordingly prevents occurrence of the combing effect that causes severe irregularity of high-frequency sound pressure response. The third function of the phasing plug is suppression of high-frequency standing waves that may occur in the compression driver.
In a horn driver, sound waves are directed to the horn through the acoustical channels of the phasing plug. The overall cross-sectional area of the channels gradually increases toward the exit of the phasing plug, finally matching the area of the horn's entrance (e.g., throat). Typically, a phasing plug configured for use in a compression driver having a dome diaphragm includes a set of concentric circular slots through which acoustic waves travel from the compression chamber to the horn entrance. The overall area of the slot entrances determines the acoustic input impedance of the phasing plug-horn combination. Maximum efficiency of the compression driver may be achieved when the output acoustic impedance of the vibrating diaphragm is equal to the loading acoustic impedance of the phasing plug-horn combination. The position and configuration of the slots in the phasing plug may help to suppress high-frequency air resonances in the compression chamber and correspondingly mitigate irregularity of frequency response at high frequencies where the radial dimension of the compression chamber is larger than the wavelength of the acoustic signal. In addition, the height (e.g., thickness) of the compression chamber separating the phasing plug and diaphragm influences the level of high-frequency signals, as the volume of air enclosed in the compression chamber is characterized by acoustic compliance that functions as a low-pass filter. As the volume of the compression chamber increases, so too does the attenuation of the high-frequency acoustical signal. The height of the compression driver is a compromise between the level of high-frequency signal and the risk of collision of the diaphragm and the phasing plug. Further, smaller compression chamber volumes (compared to the volumetric displacement of the diaphragm) are associated with higher nonlinear air compression distortion because the relationship between the variation of the compression chamber's volume and the level of the sound pressure in the compression chamber is intrinsically nonlinear.